Desirable properties of road aggregates

Road aggregates form the main part of a pavement (about 90-95% by weight), so they must have the following properties (IRC, NRS 2070 and IS:383).

1. Strength - resistance to crushing under wheel loads, particularly in the base and surface (aggregate crushing value not more than 30% for the wearing course).
2. Hardness - resistance to wear and abrasion by traffic (LA abrasion value below 30-40%).
3. Toughness - resistance to impact, in the surface and base (aggregate impact value below 30% for wearing courses; 45% for lower layers).
4. Durability / soundness - resistance to weathering, frost and chemical action (soundness loss not more than 12% with sodium sulphate).
5. Shape - angular, cubical particles that interlock; flaky and elongated particles are avoided (combined flakiness and elongation index below 30-35%).
6. Surface texture and skid resistance - a rough texture for good skid resistance (polished stone value at least 55 to 60 for the wearing course).
7. Affinity with bitumen (adhesion) - hydrophobic aggregate that resists stripping in the presence of water (basic rocks such as limestone and basalt are better than siliceous ones such as quartzite, granite or siliceous gravel).
8. Low water absorption and porosity - not more than 2% (porous aggregates absorb bitumen and are weak).
9. Good grading - proper particle size distribution for dense packing.
10. Cleanliness - free from clay, silt, organic matter and weak materials.
11. Specific gravity - sufficient (2.5-3.0) and uniform.
12. Economy and availability near the site.

Different pavement types

• Flexible pavement surface - hard, tough, skid resistant, well shaped, with good bitumen adhesion.
• Base and sub-base - strength, grading and drainage.
• Rigid pavement - good strength, low alkali reactivity, low thermal expansion and cleanliness.